The present exemplary embodiment relates generally to an apparatus and a method for driving a print head in a printing system and, more specifically, to a drive system which allows the print head to maintain alignment with a transfer surface with little or no adjustment during regular use. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Ink jet printing involves the delivery of droplets of ink from nozzles in a print head to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. The resolution of the image is expressed by the number of ink drops or dots per inch (dpi), with common resolutions being 300 and 600 dpi.
Ink jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto a final receiving medium, such as a sheet of paper. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving medium is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused or fixed to the medium. In some direct and offset printing systems, the print head moves relative to the final receiving medium or the intermediate transfer surface in two dimensions as the print head jets or orifices are fired. Typically, the print head is translated along an X-axis while the final receiving medium/intermediate transfer surface is moved along a Y-axis. In this manner, the print head “scans” over the print medium and forms a dot-matrix image by selectively depositing ink drops at specific locations on the medium.
Printers of the offset type may employ a single print head which delivers ink droplets to a drum. The drum rotates multiple times during the formation of an image. Typically, the print head includes a jetstack or plate which defines multiple jets configured in a linear array to print a set of scan lines on the intermediate transfer surface with each drum rotation. With each rotation, X-axis translation of the print head causes the jets to be offset by one or more pixels, enabling the printer to create a solid fill image, continuous line, or the like, depending on the particular combinations of jets fired.
Precise placement of the scan lines is important to meet image resolution requirements and to avoid producing undesired printing artifacts, such as banding and streaking. Accordingly, the X-axis (print head translation) and Y-axis (drum rotation) motions are carefully coordinated with the firing of the jets to ensure proper scan line placement.
As the size of the desired image increases, the X-axis movement/head translation and/or Y-axis motion requirements become greater. One technique for printing larger-format images is disclosed in U.S. Pat. No. 5,734,393 for INTERLEAVED INTERLACED IMAGING, assigned to the assignee of the present patent. This application discloses a method for interleaving or stitching together multiple image portions to form a larger composite image. Each of the image portions is deposited with a separate X-axis translation of the print head. After the deposition of each image portion, the print head is moved without firing the jets to the start position for the next image portion. Adjacent image portions overlap and are interleaved at a seam to form the composite image. In this image deposition method, the relative position of each image portion is carefully controlled to avoid visible artifacts at the seam joining adjacent image portions.
Prior art ink jet printers have utilized various mechanisms to impart X-axis movement to a print head. An exemplary patent directed to an X-axis positioning mechanism is U.S. Pat. No. 5,488,396 for PRINTER PRINT HEAD POSITIONING APPARATUS AND METHOD (the '396 patent), assigned to the assignee of the present application. This patent discloses a motion mechanism comprising a stepper motor that is coupled by a metal band to a lever arm. Rotation of the lever arm imparts lateral X-axis motion to a positioning shaft that is attached to the print head. This mechanism translates each step of the stepper motor into one pixel of lateral X-axis movement of the print head. The amount of X-axis translation per step of the stepper motor is adjustable by an eccentrically mounted ball that is positionable on the lever arm.
An exemplary patent directed to an X-axis drive mechanism is U.S. Pat. No. 6,244,686 (the '686 patent) entitled PRINT HEAD DRIVE MECHANISM, and assigned to the assignee of the present application. The '686 patent discloses a motor coupled to a lead screw by gears. While the drive mechanism of the '396 patent provides highly accurate and repeatable movement of a print head, it is nevertheless subject to minor displacement errors arising from such factors as imbalances in stepper motor phase and thermal expansion of various components under changing operating temperatures. The motor is connected with the positioning shaft by multiple gears, each gear contributing to the difficulty in maintaining tolerances. When the positioning shaft is not axially aligned with the print head, this can lead to stresses in the drive system, leading to shortened expected lifetime. Additionally, the stresses developed may cause the print head to become misaligned with the transfer drum. These misalignments tend to be of less significance when the jetstack height is relatively small.
Periodically, such offset printers are recalibrated to compensate for minor displacements in the print head or drum. In ink jet printers with a short jet array height, e.g., of about 5 mm, or less, the most sensitive alignment parameter has generally been the distance between the jetstack and the drum. Alignment is accomplished by adjustment of the print head and print engine, typically by using adjustment screws. The print head is thus fixed at a preselected spaced distance from the drum, leaving a gap between the drum and the jetstack. However, the adjustment screws do not control movement in all directions so there remains a possibility for mismatches in alignment to occur.
The present exemplary embodiment contemplates a new and improved print head drive system and method which overcome the above-referenced problems and others.